The present invention relates in general to an implant for augmenting stabilization of bone joints and in particular to an implant for augmenting stabilization of bone joints with a low relative movement and for interfragmentary stabilization of bone fractures, when primarily tensile forces are to be applied.
The joints of the body have different ranges of movement. In addition to the joints in the main axes of movement of the extremities as well as the mandibular joint, some of which have relative movements of a considerable extent, there are many joints with a low relative range of movement. Typical examples of this include the acromioclavicular joint as the connection between the shoulder blade and the collar bone (acromioclavicular joint), the joint between the collar bone and the breastbone or sternum (sternoclavicular joint), the iliosacral joint, the pubic symphysis, the articulated connections between the tibia and fibula (proximal and distal tibiofibular joints), the joints between the wrist (carpal bones) and the foot (tarsal bones) as well as the joints between the bones of the metacarpus (metacarpal joints) and those of the metatarsus (metatarsal joints). Likewise, injuries to these joints can in many cases lead to serious physical impairments, where a painful arthrosis develops as a result of a permanent joint incongruence. The therapeutic goal must therefore be to accurately reposition these joints and restore the capsule-ligament apparatus. In most cases, this cannot be accomplished by simply suturing the capsule-ligament apparatus. The sutures would not be able to withstand the stress and would rupture, and the joint would slip back into a false, incongruent position. Instead, the injured joint must be kept in a correct position through a suitable surgical implant by way of an augmentation until the capsule-ligament apparatus has healed to a sufficient level of strength and can again withstand the forces required to move the joint. The same thing is also true of unstable bone fractures, where an implant is supposed to keep the bones in the correct position, after repositioning the fracture, until the fracture has healed to an adequate level of strength.
Various techniques have been described for augmenting stabilization of a ruptured joint with small relative movements, and these techniques can be divided into roughly four groups: 1. temporary rigid bridging of the joint, 2. bridging with flexible implants, 3. retaining implants, which are bolted to one side of a joint and engage like a hook on the opposite side, 4. implants with an articular connection.
The best known representative of the first group (rigid implants) is the so-called locking screw. When using this principle, the two partners in the joint are secured rigidly relative to one another by a direct screw connection, which guarantees congruence of the joint, but blocks relative movement of the joint. Similar functions are achieved by bridging the joint with Kirschner's wires, optionally supplemented by wire cerclage or by using rigid osteosynthesis plates (especially in the area of the pelvis).
Known representatives of the second group (flexible implants) include plastic cords or bands made of absorbable or non-absorbable materials (literature: R. W. Fremerey et al. (1996) “Surgical treatment of acute, complete rupture of the AC joint,” Unfallchirurg [Trauma Surgeon] 99:341-5), wire cables in the technique proposed by LABITZKE (literature: R. Labitzke (1982) “Wire cables and intraosseous pressure distribution systems in surgery,” Chirurg [Surgeon] 53:741-3) or the use of wire cerclage.
Known representatives of the third group (screw-in implants with hooks) include hook plates proposed by Balser, Wolter or Dreithaler in a similar design for stabilization of ruptures of the acromioclavicular joint or the syndesmosis hooks developed by Engelbrecht (literature: E. Engelbrecht et al. (1971) “Syndesmosis hooks for treatment of tibio-fibular syndesmosis ruptures,” Chirurg 42:92) for stabilization of ruptures of the ankle joint. These implants allow good augmentation of the joint and essentially preserve mobility, but it is difficult to adjust the proper congruence of the joint, which can often be achieved only by bending the implant subsequently, because these implants do not have any suitable possibilities for adjustment. In addition, a relatively large surgical access area is required, which necessitates a greater surgical trauma.
A typical representative of the fourth group (implants with an integrated joint) is the joint plate developed by Ramanzadeh for stabilization of ruptures of the acromioclavicular joint. However, this plate has the disadvantage that it is difficult to adjust the correct congruence of the joint, and the axes of rotation of the joint and the implant do not match, so the natural movement of the joint is at least partially blocked.
It would therefore be desirable and advantageous to provide an improved implantable screw for stabilizing a joint or a bone fracture to obviate prior art shortcomings.